The present invention relates to an insulating substrate with increased heat radiation ability and a semiconductor device incorporating such an insulating substrate.
Semiconductor devices having a power conversion function are applied to a wide range of apparatus including consumer apparatus such as home air conditioners and refrigerators and industrial apparatus such as inverters and servo controllers.
In such semiconductor devices, insulating substrates are being developed diligently because reduction of the thermal resistance of the insulating substrate particularly contributes to improvement of the heat radiation characteristic.
A conventional technique relating to such an insulating substrate will be described below with reference to the drawings. FIG. 5 is a sectional view showing the structure of a conventional insulating substrate 500. The insulating substrate 500 is provided with a metal base 501, an insulating layer 502, and a circuit pattern 503.
The metal base 501 is an aluminum plate, an aluminum alloy plate, a copper plate, a copper alloy plate, or the like.
The insulating layer 502 is an insulating layer formed by solidifying an epoxy resin containing an inorganic filler such as silicon oxide (SiO2), aluminum oxide (Al2O3), or aluminum nitride (AlN), and currently has thermal conductivity of about 7.0-10.0 W/m·K.
For example, the circuit pattern 503 is a copper thin film pattern.
Because of its superior heat dissipation performance, the insulating substrate 500 having the above three-layer structure is used as a wiring substrate on which high heat generation components such as power semiconductor devices are mounted. In particular, the insulating substrate 500 is used as a substrate for a wiring board of a power circuit module which is used in a power supply apparatus. Since as described above its thermal conductivity is currently about 7.0-10.0 W/m·K, the insulating substrate 500 can currently be applied to a power circuit module whose current capacity is about 50 A or lower. However, it is difficult to apply the insulating substrate 500 to a power circuit module whose current capacity exceeds about 50 A.
Another conventional technique relating to an insulating substrate will be described below with reference to the drawings. FIG. 6(a) is a sectional view showing the structure of a conventional insulating substrate 600 in ordinary form, and FIG. 6(b) is a sectional view showing the structure of a conventional insulating substrate including a metal base substrate. As shown in FIG. 6(a), the insulating substrate 600 is provided with a ceramic base substrate 601, a first circuit pattern 602, and a second circuit pattern 603.
The ceramic base substrate 601 is a plate mainly made of alumina (Al2O3), aluminum nitride (AlN), silicon nitride (Si3N4), or the like, and has a thickness of about 0.2-0.8 mm. The thermal conductivity is about 20 W/m·K in the case of alumina (Al2O3), about 160-180 W/m·K in the case of aluminum nitride (AlN), and about 80 W/m·K in the case of silicon nitride (Si3N4), and hence is one or two orders higher than that of an insulating layer made of an epoxy resin containing an inorganic filler like the insulating layer 502 shown in FIG. 5.
The first circuit pattern 602 is a pattern that is formed under the ceramic base substrate 601. Being metal foil of copper or aluminum, the first circuit pattern 602 is joined to the ceramic base substrate 601 directly or with a brazing material. Usually, the first circuit pattern 602 is merely a solid plate body having no circuit pattern. The first circuit pattern 602 is grounded.
The second circuit pattern 603 is a pattern that is formed above the ceramic base substrate 601. Being metal foil of copper or aluminum, the first circuit pattern 602 is joined to the ceramic base substrate 601 directly or with a brazing material. The second circuit pattern 603 is an ordinary circuit pattern.
In the insulating substrate 600 in which the circuit patterns are formed on both sides of the ceramic base substrate 601 having the high thermal conductivity, heat generated by power semiconductor devices (not shown) mounted on the second circuit pattern 603 is dissipated via the route of the ceramic base substrate 601→the first circuit pattern 602. This enables use of a higher capacity power device and power semiconductor device whose current capacity exceeds 50 A, which cannot be realized by the above-described insulating substrate 500.
FIG. 6(b) shows another structure (module structure) in which a metal base substrate 604 such as a copper plate of 2-3 mm in thickness is joined to the insulating substrate 600 via a solder portion 605.
In this insulating substrate in which the circuit patterns are formed on both sides of the ceramic base substrate 601, heat generated by power semiconductor devices (not shown) mounted on the second circuit pattern 603 is dissipated via the route of the ceramic base substrate 601→the first circuit pattern 602→the solder portion 605→the metal base substrate 604. This enables use of a higher capacity power device and power semiconductor device whose current capacity exceeds 50 A, which cannot be realized by the above-described insulating substrate 500.
The invention of Japanese Patent Publication No. 2004-47863 (paragraphs 0026-0032 and FIGS. 1-3), for example, is another conventional technique relating to a semiconductor device in which improvement of the heat radiation characteristic is intended. This reference discloses a technique of forming a ceramic layer on one surface of a conductive substrate by an aerosol deposition method.
As described above, the use of the insulating substrate 600 incorporating the ceramic base substrate is preferable in applications in which the current capacity exceeds 50 A.
However, there is a problem that the insulating substrate 600 incorporating the ceramic base substrate is expensive for the following reason. To manufacture the insulating substrate 600, first, the ceramic base substrate 601 is formed by firing, at a high temperature, a sheet (called “green sheet”) formed by kneading a material powder and a binder together. Then, the first and second circuit patterns 602 and 603 each of which is copper foil or aluminum foil are joined to the ceramic base substrate 601 at a high temperature to complete the insulating substrate 600. As such, the insulating substrate 600 including ceramics as a material requires many manufacturing steps in forming the ceramic base substrate 601, resulting in a high price.
In the semiconductor device of JP 2004-47863, a wiring pattern is formed by an ordinary plating method or the like. There is demand that the total heat radiation characteristic be improved through improvement of the wiring pattern.
The present invention has been made in view of the above-described problems in the art, and an object of the invention is therefore to provide an insulating substrate which is inexpensive because of a reduced number of manufacturing steps, and is superior in heat radiation and insulation performance.
Another object of the invention is to provide a semiconductor device using such an insulating substrate.
Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.